In the construction of rubble masonry, less care may be taken in the exact placing of the stones, but it is highly important that all the joints be completely filled. The strength of the masonry depends upon the stone having full bearing on the mortar at all points. The interstices between large stones in rough rubble are filled by driving stone chips into the mortar.
Stratified stones should always be set upon their natural beds, and not set on edge.
Dimensions of rule frequently used is that the width of a stone shall not be less than its height. The length of the stone, to avoid danger from cross-breaking, in well-laid masonry, may be about three times the thickness for the weaker stones, and about five times the thickness for the stronger ones.
laying masonry it is not feasible to make well filled, smooth joints at the face of the wall. It is usual, therefore, after the masonry has been laid, to clear out the joint to a depth of about an inch and refill with special mortar. This is called pointing the masonry. If, in placing the masonry, the mortar in the joints is not brought quite to the face of the wall, the labor of pointing may be somewhat lessened.
The joint is cleared and brushed out to a depth of at least an inch and well moistened before applying the pointing. The mortar is then applied with a small trowel, squeezed in, and smoothed with a special tool called a jointer, which is provided with an edge to form the kind of finish desired. There are a number of ways of finishing joints, of which the most common are shown in Fig. 30.
The best pointing mortar is usually composed of Portland cement and sand, 1 to 1. Coloring matter is added when needed. The mortar is used quite dry, like damp earth. When the face of the stone would be stained by Portland cement, a putty made of lime, plaster of paris, and white lead is sometimes employed. Various non-staining cements are also available.
49. Trimmings.—In the erection of masonry structures, certain special parts are ordinarily required to be of cut stone, which must be of definite form and dimension. These trimmings have to do with the ornamentation of the structure, finishing about openings. or joining different types of construction.
with sloping surfaces are used at the top of founda tion walls, where they join the narrower upper walls.
Copings, cornices, window sills, and sometimes pro ject beyond the surface of the wall; they must have sufficient width to be firmly held in the wall, and to balance on the wall in laying. The projections should also have upper surfaces which slope away from the wall, and a drip (called the wash) underneath to cause water to drop off at the outer edge, the drip being made by cutting a groove on the under side of the stone.
When cut-stone trimmings are used for a brick wall, they should be dimensioned so that they will fit into the brickwork without splitting the brick.
Window sills just the width of the opening and not built into the wall at the ends are called slip sills, while those extending into the walls are called lug sills. The ends of lug sills are rectangular, the sloping surface of the sill being made the width of the opening. Lug sills should be bedded only at the ends to prevent cross-bending stresses due to the weight of the wall.
When stone lintels are used to span openings, care must be taken in selecting the stone, and making sure that it has the transverse strength necessary to carry the load. When necessary an angle bar or I-beam may be used to support the lintel, a recess being cut into the back of the stone for this purpose.
50. Specifications for Stone following general requirements for stone masonry and special requirements for bridge and retaining wall masonry are recommended by the American Railway Engineers' Association in their Manual for 1915: ,Stone.-3. Stone shall be of the kinds designated and shall be hard and durable, of approved quality and shape, free from seams or other imperfections. 'Unseasoned stone shall not be used where liable to injury by frost.
Dressing.-4. Dressing shall be the best of the kind specified.
5. Beds and joints or builds shall be square with each other, and dressed true and out of wind. Hollow beds shall not he permitted.
6. Stone shall be dressed for laying on the natural bed. In all cases the bed shall not he less than the rise.